Microstructure, thermomechanical and shape memory properties of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with B addition

Microstructure, martensitic transformation, thermomechanical stability and shape memory effect of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with small B addition, were investigated by thermomechanical cycling. It is found that the high degree of martensite stabilization is observed in all studied alloys due to the compressive pre-strain. The results show that B addition and thermomechanical cycling facilitate the precipitation of the ordered γʹ phase. The precipitation alters the chemical compositions of martensite and results in the decrease of martensitic transformation temperature of the alloys. As a result, the formation of the ordered γʹ precipitate has negative effect on the thermomechanical stability of the alloys. Results further demonstrate that the martensite reorientation proceed becomes easier with small B addition due to the decrease of the yield strength, which can effectively improve shape memory effect of the alloys. However, with increasing B concentration and number of cycles, the shape memory effect gradually decreases due to the combined effects of the roles of γ phase and γʹ precipitate, and thermomechanical cycling process.